Epidemiology of Stemphylium leaf spot and purple spot in no-till asparagus

A severe purple spot infection (causal agent; Stemphylium vesicarium, teleomorph Pleospora herbarum) early in the growing season can defoliate asparagus ferns prematurely, resulting in decreased yields of susceptible varieties, especially if defoliation occurs in consecutive years. Infection of spears is unsightly and commercially unacceptable. Atmospheric concentrations of conidia and ascospores of the purple spot pathogen were monitored with Burkard volumetric spore traps in two no-till asparagus fields near Hart, Michigan during the 1992 to 1994 growing seasons (April through August). Weather recording instruments in each field provided hourly records of temperature, rain, relative humidity, and leaf wetness. Ascospores from pseudothecia on overwintering asparagus debris on the soil surface appeared to be the only source of inoculum for infection of spears. The release of the ascospores responsible for the lesions observed on the spears usually was associated with rain. Purple spot disease developed on spears harvested greater than or equal to 48 hours after ascospore discharge. In general, during May and June atmospheric concentrations of conidia were low but increased later in the season. From mid-July through August, the number of leaf spots on asparagus ferns increased following peak concentrations of ascospores and conidia.